Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, a need for increased transmission rates for conveying data is emerging, whereby an occurrence of transmission errors should be minimized or maintained at an acceptable level. One currently favored technical solution to control a transmission error rate is known as the channel quality indicator (CQI).
CQI is currently used in systems implementing the 3G wireless communication standards (using e.g. wideband code division multiple access in frequency division duplex mode WCDMA FDD, time division synchronous code division multiple access TD-SCDMA and code division multiple access 1x evolution data optimized CDMA2000 1x EVDO) or future wireless communication standards 3.9G or 4G (e.g. long-term evolution LIE, long-term evolution-advanced LTE-A), and may in general be used in systems where data is transmitted in the form of packets.
Currently, in the field of mobile data transmission, a system known as high-speed downlink packet access (HSDPA) is used. The following explanation is performed on the basis of HSDPA implementing TD-SCDMA only as an example. In this system, a terminal as a user equipment (UE) communicates with a base station such as an evolved Node B, i.e. an eNodeB (eNB). Whereas the following description is performed as a method controlling the base station to terminal transmission it is understood that the same method can be used in the other direction in case a CQI based mechanism is used in data transmission from terminal to base station.
CQI is an indication of the propagation conditions (e.g. path loss, interference, noise level) of the transmission path between a transmitter and a receiver experienced by the receiver. A CQI is obtained at a UE and subsequently transmitted to an eNB. With the received CQI, the eNB is able to adjust its transmission conditions in order to maintain an acceptable transmission error rate. Further, a scheduling between a plurality of UEs to be served by the eNB can be performed.
Typically, a CQI is obtained at the UE by estimating the signal to interference ratio (SIR) of a received signal and subsequently by determination of adjustable transmission parameters based on the SIR. According to discussions with respect to the standard WCDMA FDD, thereby an estimated SIR is mapped onto a CQI index by performing the equationCQI=floor(SIR+offset[dB]).
It is noted that “offset” represents a constant value. As an example, when the constant value offset is set to 3.5 dB, for an estimated SIR of 10.0 dB, a CQI of 13 is determined. Thereby, each CQI represents a combination of transmission parameters (e.g. transport block size, modulation scheme, coding rate).
However, such processing has some associated disadvantages. Since the mapping is performed merely by adding a constant value, the SIR range is evenly represented by the CQIs, i.e. there is a 1 dB step in the SIR range between two consecutive CQI's. Each CQI is assigned to a combination of transmission parameters. Such assignment is typically implemented by a look-up table, i.e. the transmission parameters (e.g. transport block size, modulation scheme, coding rate) represented by the determined CQI can be obtained by a look-up table operation. However a dependency of the transmission parameters on the SIR is not generally linear. Thus, such a simple mapping is not suitable for each CQI implementation. For example, in TD-SCDMA, there is no fixed SIR step between consecutive CQIs and thus a simple linear mapping is not suitable. However, in TD-SCDMA a standardization of CQI mapping, i.e. determination of transmission parameters based on received signal quality, is not addressed at all.
Further, it is observed that the size of a transmission, i.e. the number of assigned resource units (RU) have an influence on the error rate of a transmission. However, such a number of RUs cannot be considered in a CQI mapping according to the example shown above. In addition, although a CQI is suitable for minimizing an error rate of a transmission to an acceptable value, there is no method known for maintaining a certain block error rate (BLER) of received transport blocks.
The above stated disadvantages point out that current implementations of the CQI cannot consider possible non-linear dependency of transmission parameters optimal for a certain SIR on said SIR. Furthermore, the size of a transmission to be transmitted is not considered for determination of optimal transmission parameters.
Hence, the problem arises, as to how actual existing propagation parameters and the size of a transmission to be transmitted can be mapped to optimal transmission parameters such as transport block size and modulation scheme used for the transmission, and how a predetermined block error rate can be maintained.
Thus, there is a need to improve techniques for determining the CQI in order to improve the adaptation of transmission parameters to present propagation conditions of the transmission path between an eNB and a UE.